Alzheimer's disease (AD) is the most common cause of dementia in the elderly. Mutations in the amyloid precursor protein gene (APP) and presenilins (1 and 2; PS1 and PS2) cause autosomal dominant, early-onset forms of AD and account for ˜1% and ˜50% of inherited cases, respectively. Polymorphisms in the apoE4 and α-2 macroglobulin genes are associated with increased risk in individuals over 60 years of age.
The presenilins are polytopic membrane proteins expressed in the endoplasmic reticulum, Golgi complex in dendrites (close to dendritic spines) and axon terminals in neurons. The PS1 holoprotein is subject to endoproteolysis; the resulting N- and C-terminus fragments bind to each other at stoichiometric levels and/or other proteins, such as γ-catenin. The levels of the fragments are very tightly regulated and overexpression studies show little changes in the relative amounts of accumulated fragments.
The normal biological function(s) of presenilins are not well understood although they have been shown to play a major role in the embryonic development of the axial skeleton and cerebral vasculature. The inheritance pattern in humans carrying mutant presenilin genes suggests a gain-of-function. Several cellular effects of mutant presenilins have been documented that may be relevant to the pathophysiology of AD. First, in cultured cells and transgenic animals expression of mutant presenilins lead to the elevated production of αβ42(43) peptides that are deposited early and selectively in amyloid plaques in AD. The over-production of Aβ42(43) is most pronounced in cells expressing a PS-1 mutation lacking exon 9 (Δ9). Secondly, cells expressing mutant presenilins also have aberrant calcium homeostasis; PC12 cells expressing mutant PS1 stimulated with agonists that activate Ca2+ efflux from intracellular stores, exhibit larger calcium transients than cells expressing wild-type (wt) PS1.
The clinical hallmark of early AD is a disruption of memory processes. The hippocampus, which is prominently involved in the formation of memory, is affected early in the disease and shows the characteristic histopathological changes of AD, namely senile (amyloid) plaques and neurofibrillary tangles. A loss of synapses is also apparent in the hippocampus early in the disease. As the disease progresses, neuronal death in the hippocampus increases (see review by Price et al., 1998 Annu. Rev. Neurosci. 21:479-505).
Hippocampal slices have been effectively used to examine synaptic transmission and plasticity in vitro. Stimulation of CA1 striatum radiatum afferent pathways produces a mixed excitatory and inhibitory synaptic response in pyramidal neurons. Brief repetitive stimulation generates a short and long-term potentiation (STP: ˜20 min and LTP: >30 min) of excitatory transmission that have been proposed as cellular correlates of some forms of leaning. STP and LTP also share several underlying mechanisms with glutamate-mediated neuron death (excitotoxicity, see reviews by Obrenovitch et al., 1997 J. Prog. Neurobiol. 51:39-87 and Choi, 1992 J. Neurobiol. 9:1261-1276). All are believed to require synaptically induced postsynaptic depolarization, activation of NMDA receptors and a rise in intracellular calcium concentration. As membrane depolarization is a critical requirement for STP and LTP, these phenomena are sensitive to pharmacological manipulations of fast inhibitory pathways via the GABAA receptor. In the CA1 region of the hippocampus, for example, a GABAA antagonist leads to more LTP, while GABAA-potentiating benzodiazepines can reduce LTP. GABAA agonists can also decrease glutamate-induced excitotoxicity.
Based on the present understanding of the etiology of AD and the neuronal mechanisms associated with AD and memory, there is a need for a diagnostic method for evaluating the potential of drugs for the treatment of AD, both prophylactically and therapeutically.